1. Technical Field
The present invention relates to sphygmographs and to signal processing methods used in such sphygmographs.
2. Related Art
Internal states of the human body can be discovered by using a photoelectric sensor that emits light at a specific wavelength and measures the reflected or transmitted light in order to analyze obtained waveforms. As one example, information related to the pulse, blood, and so on in a human body can be obtained by utilizing the light-absorbing characteristics of blood in areas of the living body in which blood is present.
For instance, if a pulse wave can be accurately captured, information regarding cardiac movement can be obtained. The pulse wave interval is sometimes used in the same manner as the RR interval of an electrocardiogram, and although the pulse wave interval is used in judging partial arrhythmia, it can also be used to derive an autonomic nerve index and determine sleep states by analyzing frequencies, if long-term data can be obtained; in addition, fluctuation in the RR interval is also sometimes used to analyze heart function.
JP-A-2008-253579 attempts to acquire a pulse peak interval that corresponds to an electrocardiogram RR interval by obtaining a sphygmograph using a photoelectric sensor. Furthermore, a noise removal process is carried out in order to remove reflected waves, notches, and so on that result in noise.
However, when, in the stated method, the signal processing, or in other words, the DC component of the photoelectric sensor output signal is unstable, such as when the measurement subject is not at rest, the noise removal is insufficient for the noise resulting from body movements; it is necessary to employ signal processing that can handle high-frequency noise, low-frequency noise, and changes in the DC component. As examples of such a signal processing method, an adaptive filter that extracts a spectrum unique to pulse waves (for example, see U.S. Pat. No. 4,955,379), applying independent component analysis (for example, see U.S. Pat. No. 6,701,170), and so on have been proposed.
However, in an integrated device (sphygmograph) that has limited processing capabilities and power performance, the signal processes described in U.S. Pat. No. 4,955,379 and U.S. Pat. No. 6,701,170 are complicated in the case where a pulse is to be measured continuously on a daily basis. Accordingly, these signal processes cannot be carried out to a sufficient degree in devices with low computational processing capabilities, such as such integrated devices.
With respect to this point, the signal processing disclosed in JP-A-2008-253579 is simple and can thus be implemented, but doing so is problematic in that sufficient processing for noise cannot be carried out, which makes the measurement results unreliable. It is therefore necessary to save a plurality of signal processing results that complement one another, but the storage capacity for saving such results is limited in integrated devices.